Target organs for the hormone, avian pancreatic polypeptide (APP) have been identified in vivo in chickens by measuring the ability of various tissues to concentrate 125I-APP in the presence and in the absence of a large excess of coinjected unlabelled APP. APP iodinated at the carboxyl terminal competes poorly with APP for tissue receptors but when iodinated at the amino terminus, organ uptake of the iodinated peptide is greater and binding of radioactivity is markedly decreased by coinjection of APP. Target organs identified by this method are spleen, bone marrow, duodenum, ileum, pancreas and probably liver and proventriculus. To further localize APP to cellular elements within the tissues of these target organs, identical in vivo experiments are in progress using injections of larger amounts of 125I-APP. Tissues collected from these experiments are being studied by radioautography. Studies are being performed in which pieces of chicken pancreas, a target organ, are incubated with varying doses of APP to measure the effect upon exocrine secretion both basal and e.g. caerulein-stimulated. Similar studies will be performed in which endocrine secretion will be evaluated. The isolation of rat PP has been facilitated by the use of antiserum prepared to the carboxyl terminal hexapeptide of bovine PP. Unlike all other available antiserum, this reacts well with rat pancreatic extracts and for the first time has enable us to monitor chromatograms of these extracts effectively.